<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Title</title>
</head>
<style type="text/css">
	*{
		margin: 0;
		padding: 0;
	}
	@media(width: 320px)  {
		html{
			font-size: 20px;
		}
	};
	#box{
		height: 2rem;
		width: 16rem;
		background: #00b3c0;
	}
</style>
<body>
<div id="box"></div>
</body>
</html>